Pentafluoroethane (HFC-125) is an important substance as a substitute for CFC or HCFC which causes ozone layer destruction, and is widely used, for example, as refrigerants, mixed refrigerants (R-410A, R-407C, R-404A), blowing agents and propellants.
A number of methods for producing HFC-125 have been known. Among them, a method in which tetrachloroethylene (PCE) is used as a starting material and fluorinated in a single-stage gas phase in the presence of a catalyst to obtain HFC-125 is practical as a commercially advantageous production method.
It is known that in such a fluorination reaction, oxygen is added to the reaction system so as to suppress deterioration of the catalyst (see Patent Citations 1 to 4). A conventional production method will be described in more detail below.
Patent Citation 1 describes that various catalysts including oxides, halides and oxyhalides of chromium can be used, and a small amount of oxygen is preferably fed so as to maintain a catalytic activity of chromium oxide, and also describes that the feeding amount of oxygen is preferably about 0.01-30% by mole, more preferably about 0.05-20% by mole, and still more preferably about 0.1-10% by mole, with respect to the total organic substances to be fed into a reactor.
Patent Citation 2 describes that a mixed catalyst of oxides, halides and/or oxyhalides of nickel and chromium supported on an aluminum fluoride carrier is used, and it is well-advised to introduce oxygen at a low concentration together with reactants under conditions where the catalyst can be contaminated, and also describes that the feeding amount of oxygen may be about 0.02-5% by mole with respect to the organic reactants.
Patent Citation 3 describes that various catalysts including oxides, halides and oxyhalides of chromium can be used, and it is well-advised to introduce a small amount of oxygen together with reactive species, and also describes that the feeding amount of oxygen may be 0.02-1% by mole with respect to a gas mixture to be introduced into a reactor. Since Patent Citation 3 describes that a HF/PCE molar ratio is not smaller than 20, the feeding amount of oxygen is at least 0.42-21% by mole with respect to PCE.
Patent Citation 4 suggests use of a nonchromium-based catalyst, because a conventionally used chromium oxide-based catalyst brings about by-products when oxygen is added, and thus the objective substance cannot be selectively obtained, and also it is difficult to maintain a catalytic activity for a long time. Patent Citation 4 describes that oxygen is added preferably at an amount of 0.1-10% by volume with respect to PCE so as to maintain a catalytic activity of the proposed nonchromium-based catalyst, and also describes Examples and Comparative Examples in which oxygen is added at an amount of 2.5% by volume, 4% by volume and 5% by volume with respect to PCE. Furthermore, according to Patent Citation 4, in Examples where PCE was fluorinated for a long time, HFC-125 was scarcely produced, and the main product was HCFC-122 or HCFC-123. Therefore, Patent Citation 4 does not substantially describe a method in which HFC-125 is produced from PCE as a raw material in a single-stage gas phase.
Patent Citation 1: U.S. Pat. No. 5,545,778 A
Patent Citation 2: JP 6-247884 A
Patent Citation 3: US 2007/0129581 A1
Patent Citation 4: JP 5-97725 A
Patent Citation 5: WO 96/11176 A1
Patent Citation 6: JP 5-146680 A
Patent Citation 7: JP 11-171806 A